Gas-metal-arc-welding (GMAW) is a process in which a consumable electrode is continuously fed into an electric arc. GMAW uses the electrical arc as the heat source for melting both the base metal to be welded and the filler metal added to the weld An inert or slightly reactive shielding gas is used to protect the molten metal from the atmosphere. The shielding gas must have sufficient flow to displace the atmosphere from the arc as well as the weld pool until solidification occurs and the metal cools to a temperature where it does not react with the high oxygen and nitrogen levels in the atmosphere. The shielding gas also ionizes to form a high-temperature plasma which carries the current A mixture of argon with additions of oxygen or carbon dioxide is generally used for welding low alloy steels. Typical additions to the inner gas are 1-5% oxygen by volume or 3-25% carbon dioxide by volume. Proper control of GMAW processes depends upon timely monitoring of the shielding gas condition.
Most GMAW is performed with a constant voltage power source, which causes the arc length to be self regulating. If some perturbation causes the arc length to increase, the following steps bring the arc length into equilibrium: the circuit resistance increases; the arc current decreases; the resulting lower current melts the electrode more slowly than the electrode feed rate and the arc length decreases to a stable length. If some perturbation causes the arc length to decrease, the circuit resistance decreases and the system returns to a balance through a sequence opposite to that stated previously.
A more comprehensive description of GMAW is included in the National Institute of Standards and Technology Publication No. NISTIR3976, authored by Heald, Madigan, Siewert and Liu, entitled "Droplet Transfer Modes for a MIL 100 S-1 GMAW Electrode," published October, 1991. This publication is hereby incorporated into the present application by reference.
FIG. 1 of the present application illustrates how the contact tube 1-6, the electrode extension 1-4 and the arc 1-7 are all elements of a GMAW electrical circuit. Changes in the resistance of any element effect the electrical impedance of that circuit. Metal transfer across the arc is characterized by repetitive events, each event modulating a circuit impedance in a characteristic pattern. The characterization of the various droplet modes and events that interfere with stable transfer, permit the voltage or current records derived from sensors 1-11 to be used to monitor the arc quality.
It is well known in welding technology to use a pulsed power source to make welds. The signal from such a source has significant current and voltage pulses which are designed to stimulate the formation and detachment of droplets at the electrode tip. The power source also has an internal logic circuit that changes the pulse frequency along with the wire feed rate. Examples of pulsed GMAW power sources are found in U.S. Pat. No. 3,864,542 to Fletcher et al. and U.S. Pat. No. 4,943,701, issued to Nakajima et al.
The aforementioned systems can employ through-the-arc sensing. This technique typically uses a low-frequency sensing strategy. For example, seam tracking algorithms look for changes in the mean current or voltage (over a period of several tenths of a second). Others, such as the system described in the patent issued to Fletcher et al. look for a peak signal over a period of time so that the correct frequency can be correlated to that peak and the power supply pulse rate adjusted to operate at that frequency. It is the low rate sampling of these conventional systems which prevents timely control of the welding process to correct flaws, and keeps these systems from being effectively used for automatic or robot-controlled welding.
The system of FIG. 1 is arranged so that computer 1-10 processes signals sampled from sensors 1-11, and makes determinations for altering the weld process by sending control signals to welding power source 1-12. In order for this to be done on an automated basis, criterion from the sampled signals has to be developed so that possible flaws in the welding process can be detected on a real-time basis and corrected in time to save the on-going welding process. Only in this manner can automated welding be carried out without an unacceptable number of bad welds.
Certain characteristics of the welding process are critical in determining if an ongoing welding process should be altered or terminated. One example is the pattern of electrical signals associated with short circuiting phenomena and globular transfer. These phenomenon provide information regarding transfer mode, droplet transfer frequency, and droplet transfer stability. The spray transfer mode (a mode with smaller fluctuations as the droplets are transferred) also has corresponding electrical signals from which weld quality information can be obtained. Detection of short circuiting is indicative of an improper transfer mode when the spray mode is desired. If welding is being carried out in the short circuit transfer mode, the frequency of the short circuits will indicate if the correct voltage or current levels are being output by the GMAW power source.
Loss of shielding integrity is another major concern in GMAW. Through-the-arc sensing is capable of detecting this problem. It has been experimentally determined that there is a gradual increase in the voltage envelope as the shielding gas quality degrades. This is followed by a period of fluctuations in the voltage as the internal logic control of the power source seeks to maintain arc stability. Another, more subtle problem, is minor contamination of the integrity of the shield gas due to a cross draft or incorrect gas mixture. When the gas shield is contaminated, the voltage record becomes ragged as the contaminant, for example nitrogen, is increased. The changes in the voltage record due to contamination of the gas shield are different from those caused other phenomena such as tube wear.
However, conventional systems are unable to utilize these distinctions. Consequently, conventional systems are unable to carry out the precise adjustments needed for the real time control required in automated welding systems.